Method of forming a high tensile stength pressure vessel



3, 1965 B. J. ALECK 3,197,851

METHOD OF FORMING A HIGH TENSILE STRENGTH PRESSURE VESSEL Filed March28, 1962 3 Sheets-Sheet l INVENTOR BENJAMIN J. ALECK BY HMM+M ATTORNEYS.

3 Sheets-Sheet 2 INVENTOR BENJAMIN J. ALECK ATTORNEYS.

B. J. ALECK METHOD OF FORMING A HIGH TENSILE STRENGTH PRESSURE VESSELFiled March 28, 1962 m av #33.

Aug. 3, 1965 FIG. 5A

Aug. 3, 1965 Filed March 28, 1962 TRUE STRESS (IN POUNDS PER) SQUAREINCH B. J. ALECK 3,197,851

METHOD OF FORMING A HIGH TENSILE STRENGTH PRESSURE VESSEL 3 Sheets-Sheet3 FIG. 6.

TRUE STRAIN (PERCENT DEFORMATION) E09 5 log log INVENTOR BENJAMIN J.ALECK iii/M44 I 5%? ATTORNEYS.

United States Patent 0 3,197,851 NET-H61) 0F F012? ENG A HIGH STRENGTHPREESURE VESSEL Benjamin J. Aleelr, .lacisson Heights, N.Y., assignor toArtie-Portland, inc, South Portland, Maine, a corporation of Maine FiledMar. 23, 1%2, Ser. No. 133,149 15 Gaines. (Cl. 29-421) This inventionrelates to metallurgical processing and particularly to a metallurgicalprocessing for forming high tensile strength pressure vessels. Moreparticularly, the present invention relates to a novel step inmetallurgical rocessing for forming ln'gh tensile strength pressurevemels from metallic materials and particularly from metallic materialshaving at least two distinct crystal structures, both of which can existat room temperature.

This application is a continuation-in-part of my earlier filedapplication Serial No. 834,439 filed by me on August 18, 1959 for Methodand Apparatus for Forming Pressure Vessels, and assigned to the assigneehereof, and now abandoned.

There are many uses for high tensile strength pressure vessels and suchuses are multiplying over the years. In a nurnber of such uses, theweight of the pressure vessel is of great importance. For instance, ifthe pressure vessel is to be carried by a man such as, for example,portable oxy-acetylene tanks, it is desirable for the pressure vesselsto be as light as possible and still meet the strength requirements.

(me especially critical use for light-weight vessels is in the field ofsolid fuel rockets, where pressure vessels are employed to store thesolid rocket fuel and to contain such fuel during ignition andcombustion thereof. During such time, the vessel is subjected toextremely high pressures. In such applications it is obvious thatfailure of the vessel itself means failure of the vessel itself meansfailure of the rocket and, accordingly, considerable strength must bebuilt into such pressure vessels. However, the weight of such pressurevessels is a vital factor in rocketry as the weight of the vessel itselfwill cut into the overall weight of the payload. Accordingly, it is mostdesirable to create a high strength pressure vessel of very lightweight. Naturally, if the material from which the material is made has avery high tensile strength, its overall strength and resistance topressure will increase for a given weight of material. Thus light weightvessels can be made if extremely high tensile strength material isavailable.

There are many metallurgical processes available toay for strengtheningsteels and other metals suitable for pressure vessels and particularlyfor solid fuel rocket casings. For instance, cold rolled stainlesssteels are available and these materials have very high tensile strengthafter having been cold rolled. However, during the fabrication ofvessels from such materials, it is generally necesary to weld the coldrolled stainless steel together to form the vessel. During the weldingoperation the metal in the zone of the welds, of course, is subjected tohigh heat which heat generally destroys the gain in strength achieved bythe cold rolling. Accordingly, the final vessel, though made out ofmaterial that generally speaking has high tensile strength, will havedistinct low tensile strength zones in the vicinity of the welds whichzones will limit the overall strength of the vessel.

When forming pressure vessels from materials that are difficul to weld,such as, for instance, titanium, it is often necessary to thicken theends of the portions of ice the vessel blank which are to be butt weldedto one another in order to get additional strength in the weld. The needfor additional strength is due not only to the d-ifiiculty of eifectinga good Weld, but also due to the fact that the welding operation causesa loss in strength previously gained by earlier employed metallurgicalprocesses. However, the technique of thickening the ends is extremelycostly because in the present day art, the thickened ends of suchvessels are formed by machining out material between the ends whereby toleave a fiaired or flanged end. This is an extremely expensive proceduredue in part to the cost of machining and also due in part to the factthat the machining removes (and thus Wastes) costly material, such astitanium.

The main object of the present invention is to provide a novel step in ametallurgical process for forming pres- 1 sure vessels which step willproduce an extremely high strength vesel for a given weight.

Still another object of the present invention is the provision of ametallur ical process for working pressure vessels whereby to convertthe metallic material forming the pressure vessel into a high tensilestrength material.

Still another object of the present invention is the provision of ametallurgical process for forming welded high tensile strength pressurevessels,

Yet a further object of the present invention is the provision of ametallurgical process for working pressure vessels made out of metallicmaterials having two distinct phases or crystal structures both of whichcan exist at room temperature.

The above and other objects, and characteristics of the presentinvention will be understood more fully from the following descriptiontaken in connection with the arcompanying drawings.

In accordance with the present invention a pressure vessel blank isformed of metal, preferably by Welding. Thereaiiter, the vessel blank isplaced in a die of larger size than the blank itself. The vessel blankis then subjected to high internal pressure being sufiicient to stretchjected to high internal pressure as by injecting or compressing a fluidtherewithin, the pressure being sufiicient to stretch the metal fromwhich the vessel blank is made. The stretching of the vessel blank willalso cause a stretching of the welds and area surrounding the weldswhereby to result in an increase in strength of the entire vessel blankincluding the weld therein.

In some instances, the pressure can be applied to stretch only the weldareas whereby to raise the strength of the weld areas.

The stretch forming technique may be used in combination with variousmetallurgical processes such as a heat treatment or precipitationhardening to further increase the strength gain in the pressure vessel.For instance, in materials exhibiting a martensitic transformation, ifthe stretch forming is performed at certain temperatures, it can beemployed to induce such martensitic transformation whereby to yield avery high tensile rength pressure vessel made in whole or part ofmartensitic material, including the weld areas.

In the drawings:

PEG. 1 is a diagrammatic view of an apparatus for rocessing pressurevessel blank at cryogenic temperatures;

FIG. 2 is a sectional View of a pressure vessel formed in accordancewith the present invention disposed Within a die;

FIG. 3 is a side elevational view of a pressure vessel which has beenformed in the die shown in FIG. 2;

' ing).

FIGS. 4A, 4B and 4C illustrate several steps in the method of forming anopen ended pressure vessel having a threaded boss at the end thereof;

FIGS. 5A, 5B and 5C are views of various steps in forming an open endedpressure vessel using a modified form of the novel method disclosedherein;

FIG. 6 is a true stress vs. true strain graph of typical ductile metals;and

PEG. 7 is an elevational view of a cylindrical Welded vessel blank.

Referring first to FIG. 7, a conventional pressure vessel blank 1% isillustrated therein. This blank may be formed of a metal and, as shown,is fabricated by taking a sheet of said material to form a centralcylinder E32 by means of a longitudinally extending weld ltld. Toenclose the vessel two vessel heads 1% and 1% are welded to the ends ofthe vessel blank 1% as by circumferential welds 119 and 112,respectively. The vessel heads 1% and 1% may be planar but, preferably,are of convex configuration.

Assuming that the vessel 1% has been fabricated from a very high tensilestrength material which was previously treated by some metal workingprocess such as ausforming or cold rolling or the like, the overallstrength of the vessel will still be far below that which would betheoretically possible from the tensile strength of the material priorto fabrication thereof. This sharp reduction in the strength of thevessel is due to the fact that when the material is welded as at theWelds 104, ill) and $112, the benefits of the metallurgical process usedto raise the tensile strength of the material prior to fabrication ofthe vessel blank 19%? are generally all or at least partly lost becauseof the heat of welding. Thus, although most of the material will beextremely strong and of high tensile strength, the welds themselves andthe zones immediately adjacent the welds will be sharply weakenedwhereby to provide zones of weakness which limit the overall strength ofthe vessel.

in accordance with the present invention the vessel blank 1% issubjected to internal pressure of sufficient magnitude to exceed theelastic limit of at least the weld areas in the material whereby tocause a stretching of the welds and areas adjacent the welds and,preferably, of the overall material itself. Such stretching, as will bedescribed in greater detail hereinafter, can improve the mechanicalproperties of the welds and of the material, or can effect a change inthe crystallographic structure of the welds or material or can be usedin combination with known metallurgical process to treat the entirevessel blank after forming whereby to insure high tensile strengththroughout the entire vessel including the welds. It will be obviousthat upon the application of internal pressure, the areas of minimumtensile strength, that is the areas surrounding and including the welds,will yield first and will continue to yield without any exceeding of theelastic limit of the stronger material unaffected by the weldingoperation until the strength of the welds has come up to the strength ofthe overall material. If pressure is continued to be applied after thispoint then the entire vessel blank including the now strengthened weldareas will continue to stretch to give additional strength to theoverall vessel.

PEG. 6 shows a true stress vs. true strain curve for a typical ductilework hardenable metal such as, for example, austenitic stainless steel.It will be noted that true stress is shown in pounds per square inch andtrue strain is shown in percent of deformation (diametral stretch- As iswell known to those skilled in the art, true stress is equal to thetensile force on the material divided by the actual area of thematerial. True stress is to be contrasted with nominal stress which isequal to the actual tensile force divided by the nominal or originalarea of the material. True strain is defined as the natural logarithm ofunit deformation, i.e. log e, which is equal to ,6 final length (1;)allant length (1.

0 original area (A.,) final areahh) It will be seen that in the portionof the curve designated by the reference character 12% the curve risesalmost vertically and substantially linearly. This portion of the curverepresents purely elastic deformation and the metal observes l-lookeslaw, that is, if the stress is relieved anywhere within this portion ofthe curve the metal will elastically return to substantially its initialshape. Hovever, once the elastic limit has been exceeded at the point122, t 1e material tends to yield with small increase in true stress,this portion of the curve being observed to be substantially horizontaland running from the point 122 to the point shown by the arrow 3.24.This approximately horizontal portion of the curve is designated by thereference character 126. In this area of the curve there is considerableyielding of the material and Hookes law is not followed. Between thepoint defined by the arrow 124 and the point defined by the arrow 123the curve shows a concave upward characteristic and it is in this areawhere work of a substantial amount commences to be put into the materialto sharply increase its tensile strength. This portion of the curve isdesignated by the reference numeral 13%. Beyond the concave upwardportion 13%, the true stress vs. true strain curve commences to follow asharp upwardly rising part which is substantially linear but slightlyconcave downward, which part of the curve is designated by the referencecharacter 132. As stress increases this part of the curve becomes moreand more horizontal but never bends downwardly due to the fact that assoon as this tendency arises, the material breaks. Generally speaking,it is preferred to stress the material during stretching thereof intothe portion of the curve part 132 between the arrows 123 and 134 whereina substantial strength gain is achieved.

' With continued reference to FIG. 6, it is presently necessary in orderto achieve substantial strength gains to cause deformation beyond thearrow 124. That is, it is necessary to stretch the vessel blanksufilciently to move at least into the concave upward portion 136 of thetrue stress vs. true strain curve. Most preferably, it is desirable tostretch the vessel beyond the concave upward portion of the curve andWell into the concave downward portion 132 of the curve. For stainlesssteels and particularly for AISI No. 301 stainless steel, the point 124is-reached at about 5% unit deformation (i.e., e=5%), and this is theminimum deformation above which any substantial tensile strength gaincan be achieved. For 301 stainless steel it is desired to stretch thematerial in the vessel blank somewhere between the 10 and 18 percentunity deformation (i.e., e=1 Q% to 18%). However, some improvement inmechanical properties of the vessel is achievable at deformations belowpoint 124 (i.e., s 5%) and, for certain applications this may bedesirable.

A number of metals display what is known as a martensitic tranformation.The mar ensitic tranformation is a rearrangement of the crystallographicstructure without any change in the chemical composition of the crystalstructure. It is diifusionless. Moreover, such transformations inmaterials in which they occur are spontaneous at certain temperatures.For instance, as the temperature of the material is dropped, atemperature point will be reached where a martensitic tranformation willcornmence occurring spontaneously. This temperature is known as the Mtemperature. ihe martensitic transformation will progress further as thetemperature of the material continues to be dropped until at a certaintemperature, generally known as the M; temperature, there will bemaximum spontaneous martensitic transformation, that is as muchmartensite as can be formed will be formed. It ha been found that themartensitic transformation can be started above M temperature if thematerial is deformed, that is. if mechanical work is put into thematerial. However. there is a maximum temperature above which nomartensitic transformation will occur even if deformation takes place.This temperature is known as the M temperature. Moreover, it has lmenfound that at temperatures below the M, temperature, the martensitictransformation can be made to progress further than it normally wouldspontaneously, provided the material is mechanically deformed at such atemperature.

ln view of this knowledge and finding. I have discovered that withpressure vessels made of materials which exhibit a martensitictransformation, it is desirable to stretch the vessel blank at atemperature below the the M temperature. and, preferably, close to theM, temperature and most preferably at or slightly below the M,temperature. when the stretching of such vessel blanks is performed atsuch temperatures. the vessel blank will gain in strength not only dueto the stretching of the material, but also due to the crystallographictransformation of the material to the generally stronger martcnsiticphase. While it is preferred to stretch form materials exhibiting themartensitic transformation at about the M, temperature at whichtemperature the material will be relatively ductile to thereby enablethe processor to substantially deform the vessel blank while formingmartensite, substantial strength gains can be achieved at temperaturesbelow the M, temperature at which there is a substantial amount ofmartcnsitc already present and even at or below the M temperature atwhich all the martcnsite formablc has been formed.

The following table lists the M, temperatures for a number of materials:

Approximate Material: M,temp.(F.) Titanium 1570 Aluminum-copper alloys(percent Al) 13.0 430 m 15.1) 150 Stcel/\1Sl N0.

4330 630 4330 .1792, v, 610 4340 5S0 Iron-nickel alloys (percent Ni)--In addition to the materials listed in the chart, the ausieniticstainless steels also exhibit a martensitie transforma- .ion and forthese steels an equation exists to calculate the approximate M,temperature. This equation is as follows:

Utilizing the above formula. it has been found that with four stainlesssteels the following occurs: With a stainless steel having the followingcomposition- Percent Cr 16.8 Ni 6.1

Mn 1.33 Si 1.49

Fe, balance.

the calculated M temperature is 71 degrees F. The measured M,temperature is 156 F. The following are other compositions of stainlesssteel with both measured and calculated M, temperatures:

Percent Cr 17.30 Ni 7.56 Mn 1.33 Si .49 C .031 N .05 Fe. balance.

A material having the above composition has an M temperature calculatedby the above presented formula of F. The M, temperatures as measured bystandard techniques is 60 F. For stainless steel having the followingcomposition- Percent Cr 16.! Ni 10.2 Mn 1.42 Si 0.46 C .023 N .042

Fe, balance.

the calculated M temperature is --297 F. and the measured M temperatureis about 320. F.

Another stainless steel has the following composition-- Percent Cr 11.7Ni 14.8 Mn 1.25 Si 0.33 C 0.052 N 0.035

Fe, balance.

This material has a calculated M, temperature of -479 F. (obviouslyimpossible) and an actual measured M, temperature of -452 F.

With all materials displaying a martensitic transformation, as alreadynoted, the stretch forming preferably takes place at a temperature belowthe M temperature and more preferably at a temperature at or below theM, temperature, whereby to effect a martensitic transformation at leastdue to deformation and, within the preferred temperature range.partially by deformation and partially by the spontaneous transformationdue to temperature alone. In any of these events, assuming that thevessel blank to be stretched has been welded, it will be seen that theweld areas will also be stretched and the martensitic transformationwill take place therein as well as in the non-weld areas of the Vesselwhereby to greatly increase the overall strength of the pressure vessel,including the weld areas.

The step of stretch forming as described hereinbeforc can also beemployed in connection with other types of metallurgical treatments thanthose necessary to effect a martensitic transformation. For instance,with ferrous materials, conventional heat treatment techniques can beemployed to effect a pearlitic transformation or a transformation tobainite, preferably a lower bainite, in conjunction with the stretchforming technique. For instance, if it is desired to stretch form apressure vessel blank and form bainite, then the pressure vessel blankwill be heated above the cqttilibrium temperature of the vessel blankmaterial and then quenched to a temperature in the isothermaltransformation temperature range of the material to form bainite, withthe lower portion of the range being desired, then maintaining thetemperature of the vessel substantially constant while stretching it.While data for the determining of isothermal temperatures of variousferrous materials necessary to form bainite are readily available frontstandard time-temperature transformation curves (T-T-T curves), as anexample, and not by way of limitation, the following table oftemperature ranges for typical materials is prescntcd.

Isothermal transformation .\faterial: temperature range Nickel steel(lI'I'E- Ni. ll C.) 700" F. to 800 F. AISA 4H) hardenable stainlesssteel 800 F. to 1300' F. AISA 43-10 steel (100 F. [0 ll00 F. AlSA i030steel 500 F.lO1l00 F.

During the isothermal period of the process, the material will transforminto bainite. Moreover. if the temperature tends to be towards thebottom of the range presented, the bainite will be what is commonlyknown as lower bainite. which is a very strong tough material.Naturally, with respect to transformation to bainite, this is onlyapplicable to ferrous materials.

From the foregoing. it will be seen that the stretch forming of pressurevessel blanks in combination with various tuetallttrgical processes willgive rise to very strong high tensile strength pressure vessels. Stretchforming is particularly useful with respect to welded vessels in view ofthe fact that the welds of the vessel are treated along with the rest ofthe material of the vessel after formation of the vessel blank wherebyto give high strength throughout the ve el. which is not ordinarilyachievable by conventional techniques in which the vessel material istreated prior to formation into a pressure vessel and the benefits ofthe treatment are lost in the vinicity of welds when welding isperformed. The gain from stretch forming may be due to an improvement ingrain structure of the vessel blanks throughout. or the increase inyield point due to working. or any of these in combination with gainscoming from known metallurgical processes.

Various techniques may be employed in stretch form ing pressure vesselblanks into high tensile strength vesscls, and these will now bedescribed in connection with the drawings and particularly in connectionwith FIGS. 1 through 5C thereof. Moreover, the method will be describedin connection with a pressure vessel blank formed of stainless steeldesignated as AlSI stainless steel numher 302 which has the followingcomposition:

Percent Carbon --ma. t- .l5 Manganese --max- 2 Silicon 1 Chromium 17 to19 Nickel 8-10 Phosphorus max .041 Sulfur ...max.. .03

Austenitic iron, balance.

Certain of the stainless steels coming within the ranges of tlte AlSlNo. 302 stainless steel have an M, temperature ln the vlelnily of -320'F. Accordlngly. lf It is desired during stretch forming to produce amartcnsltlc transformation of such stainless steel. it will be necessaryto stretch form the vessel blank preferably at the M, tempcralurc of 320F. Of course, a certain amount of martcnsitic transformation can beachieved during stretch :all

(ill

8 forming at higher temperatures up to the M temperature of the steel.However, it is preferred to work at or below the M temperature in orderto maximize the amount of martensitic transformation.

To effect stretch forming at the hi temperature of the 302 stainlesssteel, it is, of course, necessary to em ploy cooling apparatus inconnection with the stretching apparatus. which apparatus is illustratedin FIG. 1 and is generally designated by the reference character 12. Thevessel blanks which are made of 302 stainless steel are generallydesignated by the reference character 10.

The apparatus 12 includes a die 14 the internal surface of which issubstantially identical to the final surface to be achieved for thefinished pressure vessel to be formed from the vessel blank 10. Die 14is disposed within a cold chamber 16 of an insulating chest 13 having arelatively thick thermal insulating wall 20 surrounding the chamber 16.Wall 20 is provided with a door or closure 21 to provide access tochamber 16. Door 21 may be hinged as at 23 and have a handle 25. Alsodisposed within chamber 16 are cooling coils or trays 22 here shown tobe in the form of two flat coils although a helical coil may beemployed. The cooling trays 22 may be formed by taking a piece of tubingand bending it to follow a tortuous path. The cooling trays 22 areemployed to cool the die 14 to the desired working temperature which ispreferably about 320 F. in order to produce a martensitictransformation. It should be understood that cooling of the die,although advantageous, is not absolutely necessary to working thepresent invcntion as the important factor is the cooling of the vesselblank which may be achieved without a pre-cooling of the die.

To achieve the very low tetnpcratures of the order of -320 F., a coolantor refrigerant such as liquid nitrogen is preferably employed. As shownherein the liquid nitrogen coolant is contained in a tank 24 having anoutlet 26 which is connected to a conduit 28 for conveying the liquidnitrogen out of the tank 24. A valve 30 may be interposed in the conduit28 to control. the flow of the liquid nitrogen. As shown herein theliquid nitrogen is supplied to the cooling trays 22 by a branch pipe 32having a control valve 34 interposed therein. The conduit 28 is providedwith a second branch pipe 36 which goes to the intake of a pump 38having an outlet 40 the flow through which is controlled by a valve 42.The outlet pipe 40 extends through the insulating wall 20 of the coolingbox 18 and into the chamber 16. in the chamber 16 the pipe 40 passesthrough an opening 44 in the die H and is threadedly connected to thepressure vessel blank 10 at a threaded inlet 46 therein. The die 14 hasan opening 48 at its other end to which is connected an outlet pipe 50having a valve 52 interposed therewithin to control the flow of fluidstherethrought. If desired, the pipes 32. 36 and 40 may be provided withcheck valves 54 to limit the pressure therewithin. Furthermore,temperature measuring instruments 56 may be included'to monitor thetemperature of various parts such as the die, the trays and the chamber.Moreover. if desired. a bleed valve 58 may be included in communicationwith the pipe 40 to relieve the pressure in the pressure vessel 10 atthe end of the pressure step as will be described hereinafter. Toprevent the refrigerant from picking up heat, pipes or conduits 28, 32,36 and 40 may be thermally insulated as by insulation 59.

' To form a pressure vessel in accordance with the presu nltt'ogen fromthe latter two plpes.

.3 t0 the central cylindrical section 62 thereof by threaded ecuringelements (not shown) which may be removed to permit the detachment ofthe hemispherical section 60 from the remainder of the die 14. Thereuponthe pressure vessel blank 10 may be disposed within the interior of thedie 14. The method of forming the pressure vessel blank 10 will bedescribed hereinafter in greater detail. Suffice it to say at this timethat the outer shape of pressure vessel blank 10 is of smaller size thanthe interior surface of the die 14. With the pressure vessel blank 10disposed within the die the pipe 40 may be connected to the inlet 46 andthe removable hemispherical section 60 may be secured to the remainderof the die 14 to thus close the die. Thereafter, the door 21 may beclosed.

After the completeion of the insertion of the blank 10 in the die 14,the valve 30 may be opened and the valve 34 may be opened to permitliquid nitrogen in the tank 24 to flow under its own pressure into thecooling trays 22 to thus lower the temperature of the die 14. At thesame time or shortly thereafter the pump 38 may be energized and thevalve 42 may be opened to permit liquid nitrogen under a pressure of theorder of 2000 to 2500 p.s.i. to be fed through the pipe 4%) into theinterior of the pressure vessel blank 10. With the liquid nitrogen beingsupplied to the interior of the pres sure vessel blank as well as thecooling trays the temperature of the die and the pressure vessel blank10 will rapidly drop to approximately the temperature of the liquidnitrogen, that is 320 F. Moreover, with the liquid nitrogen beingsupplied to the interior of the pressure vessel blank 10 at highpressure the liquid nitrogen will force the pressure vessel blank to bestretched outwardly to conform the outer surface thereof to the innersurface of the die 14. This stretching may be of the order of 10% to 20%which is well beyond the elastic limit of the vessel blank materialthereby resulting in a plastic deformation of the vessel blank. Thisdegree of deformation at the working temperature of -320 P. will cause asubstantial amount of transformation to the martensitic phase to producea substantial gain in stength over and above that produced by workhardening. Accordingly, when the pressure on the inside of the pressurevessel blank 10 is relieved the pressure vessel blank will not return toits original shape but will, save for a slight elastic shrinkage,substantially conform to the shape of the interior of the die 14.

Accordingly, after the application of the pressure on the interior ofthe vessel by introducing liquid nitrogen therein under pressure, thepressure may be relieved by shutting off the pump 38, closing the valve42 and opening the relief valve 48 which will vent the liquid nitrogenin the tank to atmosphere, At the same time the valve 39 may be closedto discontinue supplying liquid nitrogen to the cooling trays 22.Thereafter, the door 21 in the wall 2t? may be opened, the hemisphericalsection 6% of die 14 may be removed from the remainder of the die andthe pressure vessel, now stretched fully to size, may be removed fromthe remainder of the die and there after the method may be repeated.

It will be understood that the pressure vessel, after being stretched tosize, may be readily removed from the die as after the pressure isrelieved from the interior of the vessel, the vessel will shrinkslightly within the elastic range and thus pull away from the Wall ofthe die.

The purpose of the vent 5% should be explained. As the die 14 isrelatively well sealed, and as there is air between the outer surface ofthe pressure vessel blank and the inner surface of the die, whenpressure is applied to the interior of the vessel blank It? to cause astretching thereof the air in the space between the blank and the diewill be compressed. Unless this air is permitted to flow out between thespace it will cause some malshaping of the final pressure vessel.Accordingly, the

l0 vent Si is provided to permit the air to flow from the space betweenthe vesssel and the die to the atmosphere through the valve 52.

The shape of the initial vessel blank 10 is of some importance inachieving a pressure vessel of the desired qualities. A shown in FIGS, 1and 2 the vessel blank 10 has a uniform Wall thickness and is made oftwo hemispherical end sections which are Welded or otherwise connectedto a cylindrical center section. While this particular shape will worksatisfactorily for the formation of a closed ended high strengthpressure vessel it can be demonstrated that hemispherical sections andcylindrical sections are stretched at difierent rates under uniformpressure. Specifically, the hemispherical sections cannot be stretchedlinearly as much as the cylindrical section for the same percentincrease in area. Accordingly, in lieu of the shape of the blank 19shown in fragmentary view in FIG. 1 a blank perhaps of the shape of adog bone would be preferable in order to end up with a vessel havinguniform strength throughout. Other shapes of vessel blanks may becalculated depending upon the desired shape of the end product.

While the method described hereinbefore is eminently suited to forminghigh tensile strength pressure vessels having closed ends and uniformwall thicknesses, it cannot be used readily as such to form an openended vessel. If an attempt were made to form an open ended vessel bymaking a vessel blank similar in configuration to but smaller in sizethan the open ended vessel desired, when pressure is applied to theinterior of the blank the open end would stretch and upon stretchingwould break any seal theretofore made to contain the pressure. With theseal broken, pressure would be lost and there would be relatively littlestretching accomplished.

Accordingly, additional steps must be performed in order to manufacturean open ended high tensile strength vessel in accordance with thepresent invention. The simplest method of accomplishing this is to forma ves sel blank having closed ends as has been described hereinbefore.The closed ended blank is then stretch formed in accordance with theaforedescribed method and the final vessel coming out of the die will beof a shape equivalent to the shape of two open ended vessels with theiropen ends joined together. Thereafter, the stretch formed vessel (whichis really two connected open ended stretch formed vessels) may be cutalong the line of connection of the two open ended vessels. Aftercutting, there will be two vessels of the desired shape. This isillustrated in FIG. 3 of the drawings, wherein a finished stretch formedclosed ended vessel 10' is shown. The dotted line 64 in FIG. 3 is theline of juncture of two identical open ended vessels. Accordingly, whenthe stretch formed vessel it? is cut along the line 64 two open endedvessels will be produced.

In lieu of simultaneously forming two open ended vessels as described inthe preceding paragraph, a single open ended vessel can be so formed byfabricating a close ended vessel blank and stretching it as describedabove to the desired form except for the inclusion of the unwantedclosed end. After stretching at sub-zero temperatures, the unwanted endcan be removed as by cutting or otherwise to yield the desired openended vessel.

Often times it is desirable to form a high tensile strength pressurevessel which has a thickened Wall portion. When such a vessel is desiredthe method described in connection with FIG. 1 must be modified, as witha portion of the wall of the vessel blank thicker than the remainder ofthe wall there may not be uniform stretching when pressure is applied.To overcome this ditdculty supplementary means for forcing out orstretching the thickened Wall portion can be employed. Such a means maybe, for instance, a multi-armed jack 74 as shown in FIGS. 4A and 4B. inlieu of the jack 74, having a multiplicity of angularly relateddistendable arms, a plurality of angularly related jacks may beemployed. Re-

pad

.5. ferring now to FIGS. 4A and 4B the vessel blank 19" is shown havingtwo hemispherical end sections 65 and 68 of substantially uniformthickness and a central cylindrical section '70 having an inwardlydirected flange 72. If such a vessel blank were placed in a die 14" andif pressure were applied to the interior of the vessel blank to stretchit to conform to the inner surface of the die 14" the relatively thinwalled sections of the vessel blank ill" would stretch out to conform tothe die. But, the thickened wall portion represented by the flange 72would resist such deformation. Hence, the end product would not be ofthe desired shape.

To overcome this, while the vessel blank ill?" is being formed themulti-armed jack 74 is disposed within the blank in operative engagementwith the flange 72. After completion of the vessel blank 1d" the blankmay be disposed within the die 14" with the inlet for the pressure fluidconnected to the pipe 49'. The vessel blank may then be cooled either bycooling the die by such means as the cooling trays 22 or by introducingrefrigerant into the interior of the vessel blank. After the cool-' ingof the vessel blank to the temperature as hereinbefore described, thejack 74 is operated to distend its arms and thus stretch the thickportion of the wall i.e. flange 72, of the vessel blank 16''.Thereafter, pressure can be applied to the interiaor of the vessel blankto stretch the thin wall portions as hereinbefore described. In this waya pressure vessel having a portlon of its wall of thicker dimension thanthe remainder can be formed.

If it is desired to form an open ended pressure vessel having a thickwall portion as shown in FIG. 4C then the finished vessel shown in FIG.43 may, after removal from the die, be cut through its center section toyield two finished open ended pressure vessels as shown in FIG. 4C.Often, the internal flange is utilized in open ended vessels to providea threaded connection. Accordingly, in FIG. 4C, the flange 72 is shownprovided with a thread 73.

It is possible to form a single open ended pressure vessel in accordancewith the present invention. The open ended vessel may have a thickenedwall portion or it may be provided with a uniform wall thicknessthroughout. The method of forming a single vessel will be substantiallythe same in either event. Such a method is illustrated in FIGS. 5A to SCwherein a vessel blank 143" having a flange 7a is shown. The flange 75is adjacent the open end '77 of the vessel. In forming an open endedpressure vessel from an open ended pressure vessel blank 10", the vesselblank, after formation, is first placed in a die 14"", the interior ofwhich is formed to the shape desired for the final pressure vessel. Thevessel blank is then cooled, preferably by cooling the die althoughdirect cooling may be employed. After cooling the open end 77 isstretched by mechanical means to cause it to assume the shape designedfor it in the final stretched vessel. The remainder of the vessel blank16" at this point remains unstretched. However, with the open end 77 nowstretched to its final shape, a seal may be effected with the open endto subject the remainder of the vessel blank to stretching pressures,which seal will not be lost as the end 77 cannot stretch any further tobreak the seal. Accordingly, when'pressure is applied, the remainder ofthe vessel blank will be stretched to cause it to conform to the shapeof the die and thus form a finished open ended vessel.

Referring now particularly to FIGS. 5A, 5B and 5C a vessel blank 19"having an open end 77 is shown. As shown herein the vessel blank ldhas'a flange 76 adjacent the open end although, as indicatedhereinbefore, this method is applicable to open ended blanks havinguniform wall thicknesses as well. The vessel blank 16" as described isplaced in die 1 5' the internal surface of which conforms to the finalshape of the vessel. The vessel blank 10 is then cooled. Preferably, al-

though not necessarily, cooling is accomplished by cooling the die as byintroducing refrigerant into cooling trays such as the cooling trays 22as shown in FIG. 1. When the vessel blank has been cooled to the desiredsub-zero temperature the open end 77 is stretched by mechanical means.Such a means may be a multi-armed jack like the jack 74 shown in FIGS.4A and 4B. In lieu thereof a tapered mandrel may be inserted into theopen end '77 of the cooled vessel blank and a force applied to the shaft89 connected to the mandril 78 to force the mandrel inwardly of theblank and thus stretch out the open end '77. With the outer wall of theopen end of the vessel blank stretched to engage the die a seal may beeffected as by a plate 84 and an O-ring 86. Plate 84 is preferablyprovided with an inlet opening 88 to which the pipe 40" may be connectedfor conveying the pressure medium such as, for instance, liquid nitrogenfrom a pump to the interior of the vessel blank. After connection, thevessel blank may be stretched by introducing the pressure medium intothe interior of the blank as hereinbefore described. After applicationof pressure to stretch the remainder of the vessel blank the pressuremay be cut OE and the stretched vessel may be removed from the die andit will be in the proper form and of extremely high tensile strength.

In connection with the modification described above with regard 'toFIGS. 5A to SC, the mandrel 7 3 may itself be employed to effect theseal of the open end. In such an event the mandrel should be providedwith an opening to permit the pressure medium, preferably therefrigerant under pressure, to be introduced into the interior of thevessel blank after stretching of the open end. Moreover, if a thickenedwall portion is located at other than the open end, a suitablestretching means, such as multiarmed jack 74, may be employed to stretchsuch thickcued wall portion separately from the stretching of the openend of the vessel and the thin wall portion thereof.

While it is presently preferred to apply the internal pressure to thevessel blank by means of the refrigerant under pressure as has beendescribed in detail hereinbefore, the present method can be worked byutilizing other means of applying pressure to the interior of the vesselblank. For instance, explosions within a vessel blank may be employed tosupply'the necessary pressure for stretching the vessel blank to conformto the interior of the wall of the die. However, the advantages ofutilizing the refrigerant itself are great. By utilizing the refrigerantitself as the vehicle or means for applying pressure to the interior ofthe vessel blank a very rapid cooling and an assurance of a maintenanceof the vessel blank at very low temperatures is achieved. As a matter offact, if the refrigerant serves as the pressure vehicle, it may bepossible to eliminate die cooling means such as trays 22 and still coolthe vessel blank to the desired temperature. Accordingly, it may bepossible to reduce the cost of the equipment for working the presentinvention. Moreover, there is no danger of any corrosive condensationsas migl possibly occur if other fluids are injected into the interioror" the vessel blank under pressure to effect the stretching.Furthermore, it will be apparent that some fluids, such as, forinstance, water, are inconvenient to use as they might freeze and thusprevent the application of the in ternal pressure. For these reasons theuse of the refrigerant as the pressure means is the preferredembodiment.

In the example given above, both the weld area and the remainder of thevessel blank were stretched upon the application of pressure to theinterior thereof. However, it will be understood that a vessel blank canbe fabricated from very highly tensile strength metal, the method offabrication of the blank being as by welding. However, upon the weldingbeing employed to form the vessel blank, the benefits of the priormetallurgical processing for yielding the high strength material will belost in the weld areas due to annealing in such areas, whereby to rendera vessel blank made generally of high tensile strength material but withweakened areas in and around the welds. With such a vessel, all that isnecessary to do is to stretch the weld areas to increase their strengthwith no need for stretching the remainder of the material making thevessel blank. Naturally, in view of the fact that the welds and the areasurrounding them are substantially weaker than the remainder of thevessel blank, the welds will stretch first whereby to give the resultdesired.

One of the advantages of stretching only weld areas is realized whenextremely large vessel blanks are to be formed. Generally speaking, thecost of building dies to house huge pressure vessels as might beemployed as solid fuel rocket casing for an intercontinental ballisticmissile will be so great as to render the process not readily useable.Accordingly, when fabricating such a vessel blank by using stretchforming techniques, what can be done is that a plurality of individualsections can be formed of very high tensile strength material such ascryogenically rolled stainless steel. Each of the sections isessentially cylindrical. The sections can then be joined together as bywelding whereby to weaken the material in the area of the welds but torender a very large vessel rimarily made of extremely strong material.Thereafter, without the use of a die, pressure can be applied to theinterior of the vessel so fabricated to stretch the weld areas andthereby gain strength in the weld areas. If it is desired to stretch theweld areas to effect a martensitic transformation of the material in theweld zones, then local temperature control in the area of the welds canbe effected to bring the weld areas close to the M temperature of thematerial prior to stretching whereby to induce a substantial degree ofmartensitic transformation upon stretching the weld areas. Accordingly,if the material from which the Vessel was made is an A181 302 stainlesssteel, it will be necessary to cool the weld areas by liquid nitrogen orsimilar cryogenic material in order to bring the stainless steel in theweld areas down to its iv temperature.

However, it will be seen that this type of stretching will not require adie and can be done out in the open with relatively simple equipment.

Furthermore, it is possible to have high tensile strength pressurevessels to which additional hardware must be secured after formation.Ofttimes it is necessary to secure the hardware by welding or otherprocess employing great heat, which process will tend to anneal thevessel material in the area in connection with the additional hardwarAccordingly, in order to bring the vessel back up to strength aftersecuring the hardware, it will be necessary only to stretch form thevessel blank in the area where hardware has been secured. Thus, whenworking on very large vessels, or it may be necessary only to isolatecertain portions of them for stretching whereby to cut down the amountof work to be done, the size of equipment, and so forth.

While I have herein shown and described the preferred form of thisinvention and have suggested several modifications therein, otherchanges and modification may be made therein within the scope of theappended claims without departing from the spirit and scope of thisinvention.

Having thus described my invention, what I desire to secure and claim byLetters Patent is:

l. The method of forming a high tensile strength pressure vessel from ametallic material having a true stress v. true strain curve with aconcave upward portion therein, comprising the steps of forming a vesselblank from said metallic material, and then stretching said vessel blankat least to strain said material within the concave upward region ofsaid curve.

2. The method of forming a high tensile strength pressure vessel from ametallic material having a true stress v. true strain curve of such typethat as true strain increases true stress first rises substantiallyrapidly and linearly, then remains substantially constant, then inidcreases in accordance with a concave upwardly curved relationship andthen increases in accordance with a concave downwardly curvedrelationship, comprising the steps of forming a vessel blank from saidmetallic material, and then stretching said vessel blank to strain saidmaterial into the concave downward portion of said curve.

3. The method of forming a high tensile strength pres sure vessel from ametallic material having a true stress v. true strain curve with aconcave upward portion therein, comprising the steps of forming a vesselblank from said metallic material, at least in part by welding, and thenstretching said vessel blank including the weld therein at least tostrain said material within the concave upward region of said curve.

4. The method of forming a pressure vessel, comprising the steps offorming a vessel blank from metallic material which exhibits amartensitic transformation, and then stretching said vessel blank at atemperature below the M temperature of said metallic material, wherebyto effect a martensitic transformation of at least a portion of saidmaterial.

5. The method of forming a pressure vessel, comprising the steps offorming a vessel blank from metallic material which exhibits amartensitic transformation, and then stretching said vessel blank at atemperature about the M temperature of said metallic material, wherebyto effect a martensitic transformation of at least a portion of saidmaterial.

6. The method of forming a pressure vessel, comprising the steps offorming at least in part by welding a vessel blank from metallicmaterial which exhibits a martensitic transformation, and thenstretching at least the weld in said vessel blank at a temperature belowthe M temperature of said metallic material, whereby to effect amartensitic transformation of at least a portion of said material.

7. The method of forming a pressure vessel, comprising the steps offorming at least in part by welding a vessel blank from metallicmaterial which exhibits a martensitic transformation, and thenstretching at least the weld in said vessel blank at a temperature aboutthe M temperature of said metallic material, whereby to effect amartensitic transformation of at least a portion of said material.

8. The method of forming a pressure vessel, comprising the steps offorming at least in part by welding a vessel blank from metallicmaterial which exhibits a martensitic transformation, and thenstretching at least the weld in said vessel blank at a temperature belowthe M temperature of said metallic material, whereby to effect amartensitic transformation of at least a portion of said material.

9. The method of forming a high tensile strength metallic pressurevessel by stretching at a temperature at which a metallurgicaltransformation occurs in the metallic material of said vessel,comprising the steps of forming a pressure vessel from said material andinjecting a fluid at said temperature and under pressure to bring saidvessel blank to said temperature and to stretch said vessel blank.

19. The method of forming a high tensile strength hollow article ofpredetermined size and shape and having at least one open end in a diehaving a portion of said predetermined size and shape, comprising thesteps of forming from metallic material which exhibits a martensitictransformation a closed ended hollow article blank having a portion ofsmaller size than said portion of said die, placing said article blankwithin said die with said portion of said article blank within saidportion of said die, adjusting the temperature of said article blank tobelow the M temperature thereof, applying fluid pressure to the interiorof said article blank to stretch said portion of said article blank tosubstantially conform to said size and shape of said portion of said diewhile said blank is below said M temperature, removing said stretchedarticle blank from said die, and removing from said stretched articleblank the material other than that forming said portion thereof.

11. The method of forming a high tensile strength vesselof predeterminedsize and shape and having at least one open end in a die having aninternal size and shape substantially equal to the size and shape of twoof said vessels disposed in open end to open end relation, comprising.the steps of forming from metallic material which exhibits amartensitic transformation a closed ended vessel blank of smaller sizethan the interior of said die, placing said vessel blank within saiddie, adjusting the temperature of said article blank to below the Mtemperature thereof, applying internal fiuid pressure to the interior ofsaid vessel blank to stretch said blank to substantially conform to saidinterior of said die while said blank is below said M temperature,removing said sretched vessel blank from said die, and dividing saidresulting closed ended vessel along its central transverse plane tothereby form two open ended vessels of said predetermined size andshape.

12. The method of forming a high tensile strength vessel ofpredetermined size and shape and having at least one'open end, in a diehaving an internal size and shape substantially equal to the size andshape of said vessel comprising the steps of forming from metallicmaterial which exhibits a martensitic transformation a single open endedvessel blank of smaller size than the interior of said die, placing saidvessel blank within said die, adjusting the temperature of said articleblank to below the M temperature thereof, by mechanical means stre chingthe part of said vessel blank adjacent said open end to cause said partto substantially conform to the size and shape of the part of the dieconfronting said part of said vessel blank while said blank is belowsaid M temperature, sealing said open end of said vessel blank, andapplying fluid pressure to the interior of said vessel blank to stretchthe remainder of said vessel blank to substantially conform to theremainder of said die while said blank is below said M temperature.

' 13. The method of forming a high tensile strength ve sel ofpredetermined size and shape and having a wall with a relatively thickportion and a relatively thin portion in a die having a size and shapesubstantially equal to said predetermined size and shape of said vessel,comprising the steps of forming from metallic material which exhibits amartensitic transformation a vessel blank having a size smaller than theinterior of said die and having thick and thin Wall portions, placingsaid vessel blank in said die, adjusting the temperature of said articleblank to below the M temperature thereof, by mechanical means stretchingsaid thick wall portion of said blank to substantially conform to theportion of said die confronting said thick wall portion wtnie said blankis below said M temperature, applying fluid pressure to the interior ofsaid vessel to stretch said thin wall portion of said vessel blank toconform to the portion of said die confronting said thin wall portionwhile said blank is below said M temperature.

14. The method of forming a high tensile strength pressure vessel from ametallic material having a true stress v. true strain curve with aconcave upward portion therein, comprising the steps of forming a vesselblank from said metallic material, and then stretching said vessel blankby applying a fluid pressure to the interior of said vessel lank atleast to strain said material within the concave upward region of saidcurve.

15. The method of forming a pressure vessel, comprising the steps offorming a vessel blank from metallic material which exhibits amartensitic transformation, and then stretching said vessel blank at atemperature below the M temperature of said metallic material, bycomprising a fluid at said temperature inside said vessel blank, wherebyto effect a martensitic transformation of at least a portion of saidmaterial.

References Qited by the Examiner UNITED STATES PATENTS 2,337,247 12/43Kepler 29-446 X 2,503,191 4/50 Branson 29-421 2,579,646 12/51 Branson29-421 2,861,530 11/58 Macha 113-44 2,866,429 12/58 Staples 113-443,024,938 3/62 Watter 29-4711 3,064,344 11/62 Arne 29-421 3,068,56212/62 Long 29-421 waits roan A. WlLTZ, Primary Examiner.

THOMAS H. EAGER, Examiner.

1. THE METHOD OF FORMING A HIGH TENSILE STRENGTH PRESSURE VESSEL FROM AMETALLIC MATERIAL HAVING A TRUE STRESS V. TRUE STRAIN CURVE WITH ACONCAVE UPWARD PORTION THEREIN, COMPRISING THE STEPS OF FORMING A VESSELBLANK FROM SAID METALLIC MATERIAL, AND THEN STRETCHING SAID VESSEL BLANKAT LEAST TO STRAIN SAID MATERIAL WITHIN THE CONCAVE UPWARD REGION OFSAID CURVE.